A rinsing and cleaning apparatus (RCA) is a necessary equipment in the fabrication of semiconductor devices. A rinsing and cleaning step is almost always required after the semiconductor wafers have been subjected to an etching process, and typically before they are subjected to other fabrication processes such as diffusion, thermal oxidation, chemical vapor deposition (CVD), etc. The main purpose of rinsing and cleaning is to remove the contamination, particles, organic molecules, metal ions, etc., that maybe left on the wafer surface before subjecting the wafer a subsequent process. Inadequate rinsing and cleaning can lead to poor yield and low product quality problems. However, due to the constant pricing pressure present in the semiconductor industry, semiconductor manufacturers must balance the thoroughness of the rinsing and cleaning procedure relative to the cost thereof. The completeness of rinsing and cleaning, become more critical during the fabrication of ULSI devices where there exists substantially less room for error.
FIG. 1 shows an illustrative schematic diagram of a typical wet washing station in a rinsing and cleaning apparatus. It includes an I/O Station for loading and unloading wafer to be wet washed. A robot 12 transports the wafer into various modules, such as BHF (buffered hydrogen fluoride) module, SC1 (standard clean 1) module, SC2 (standard clean 2) module. The BHF module is effective in removing oxides, SC1 (which typically contains a mixture of NH.sub.4 OH, H.sub.2 O.sub.2, H.sub.2 O) is provided to remove particles, and SC2 (which typically contains a mixture of HCl, H.sub.2 O.sub.2, H.sub.2 O) is provided to remove metal impurities. Each of the modules of BHF, SC1 and SC2 also contains a QDR (quick dump rinse), which is a water tank for a quick dump rinse of the wafer after going through the each wet wash main module. After the programmed washing steps, the wafer is sent to a dryer, where the wafer is dried, and finally back to the I/O station, where the dried wafer leaves the wet station. The sequence among the various washing modules can be programmed and can be out of order. However, in order to avoid undried wafer leaving the washing station, once the wafer enters the dryer, it cannot return to the washing modules, and must exit the wet washing station after the drying step.
The improved method and apparatus of the present invention were discovered when the co-inventors were investigating a yield problem during the fabrication of semiconductor devices which contain conductive channels (i.e., plugs) that are formed through a dielectric layer to electrically connect two conductive layers sandwiching the dielectric layer. After extensive and careful studies, the co-inventors of the present invention discovered that a native oxide layer can be formed on the surface of the bottom conductive layer, causing a high contact resistance to be present in the plug to be subsequently formed. The native oxide layer was formed due to the use of oxidizer such as hydrogen peroxide in the washing modules which released nascent oxygen atoms. The nascent oxygen atoms would react with the silicon atoms and form the native oxide layer, which is non-conductive.
One possible solution to this problem is to move the BBF module to the end of the wet washing process, making it a BHF-last wet washing station, so as to remove the native oxide. Since the native oxide layer is very thin, the BBF module is typically modified to become a diluted hydrogen fluoride (DHF), so as not to cause other complications. However, another problem arises when a DHF module is used. This occurred because the water-soaked wafer can dilute and adversely affect the strength of the DHF module, rendering it largely ineffective. If a higher strength BHF is used, it can result in different etching rates along the length of the channel, higher etching rate in the top (where etching is not required) and lower etching rate in the bottom (where high etching strength is required). In certain situations, for example, when multiple channels are formed in a relatively close area, short circuiting may occur, resulting in a yield failure.